Recently, there has been an increasing interest in an energy storage technology. As electrochemical devices are extensively applied to mobile phones, camcorders, and notebook computers, and further to electric vehicles, a research and development is conducted on the electrochemical devices more deeply. The electrochemical devices are one of the subjects of great interest in this aspect, and in particular, development of rechargeable lithium secondary batteries becomes the focus of attention.
Among currently used secondary batteries, lithium secondary batteries developed in early 1990's have a higher drive voltage and a much higher energy density than those of conventional batteries using a liquid electrolyte such as Ni-MH batteries, Ni—Cd batteries, H2SO4—Pb batteries, and the like, and thus, they arouse interest.
A variety of electrochemical devices such as lithium secondary batteries have been produced from many companies, and each exhibits different safety characteristics. Thus, the most important consideration of electrochemical devices is safety. In case of malfunction, the electrochemical devices should not cause any damage to users. Taking this into account, safety regulations strictly prohibit safety-related accidents of electrochemical devices such as firing, smoke emission, and the like. According to the safety characteristics of electrochemical devices, explosion may occur when an electrochemical device is overheated and subject to thermal runaway, or when a separator is punctured. In particular, a short circuit may occur between a cathode and an anode, when a polyolefin-based porous substrate that is commonly used as a separator of electrochemical devices shows a significant thermal shrinking behavior at a temperature of 100° C. or above due to its material characteristics and process characteristics such as elongation.
In order to solve the above safety-related problems of electrochemical devices, Korean Patent Publication No. 10-2007-0000231 suggests a separator 10 having a porous coating layer formed by coating at least one surface of a porous substrate 1 having a plurality of pores with a mixture of inorganic particles 3 and a binder polymer 5 (the FIGURE). In this separator, the inorganic particles 3 in the porous coating layer formed on the porous substrate 1 serve as a kind of spacer that keeps a physical shape of the porous coating layer, so the inorganic particles 3 restrain thermal shrinkage of the porous substrate when an electrochemical device is overheated. The binder polymer 5 binds the inorganic particles 3 to each other and secures the inorganic particles 3 contacting with the porous substrate 1 to the porous substrate 1.
To enable the porous coating layer formed on the separator to restrain thermal shrinkage of the porous substrate as mentioned above, the inorganic particles should be sufficiently included in the porous coating layer above a predetermined content. However, the higher content of the inorganic particles, the lower content of the binder polymer. As a result, the inorganic particles of the porous coating layer may be detached due to stress occurring during assembly of the electrochemical device including winding and the like. The detached inorganic particles act as a local defect of the electrochemical device, and may give a bad influence on the safety of the electrochemical device. Accordingly, there is a need for development of a binder polymer capable of reinforcing the adhesive strength of a porous coating layer to a porous substrate.
Meanwhile, when a porous coating layer has a low packing density, the porous coating layer should be formed thicker sufficiently to perform a function of the porous coating layer. As a result, there is a limitation in reducing the thickness of a separator to increase the capacity of an electrochemical device.